Optical communication systems are commonly used for exchanging information via visible and infrared light. For long distance communications, these light signals are often transmitted over a fiber optic cable. Planar lightwave circuits (PLCs) have been developed using semiconductor manufacturing technology to form planar optical waveguide structures on a planar substrate. PLCs are useful for transmitting and manipulating light signals over short distances. PLCs are typically formed as multi-layer structures by applying silicon dioxide (SiO2) glass, and/or other materials, to one or more sides of a planar silicon (Si) substrate. Since the glass layers are often produced and/or annealed at high temperatures, any difference in thermal contraction (or expansion) between the various layers can cause warping, and possibly fracture, of these otherwise flat planar structures.
The problem of warping and/or fracture may be better understood with reference to various mechanical engineering terms. Stress is a force per unit area that acts on a material and tends to change the dimensions of that material by compressing it, stretching it, or causing it to shear. Stress is commonly denoted by the Greek letter sigma, “σ.” Strain is a change in the dimensions of a body in response to an applied stress. Strain is typically expressed as the ratio of the distortion of a dimension to some undistorted dimension, and is represented by the Greek letter epsilon “ε.” Strain is said to be elastic when the deformation disappears as the stress is removed, and is said to be plastic when the deformation is permanent. Compressive strain occurs when the body dimension is reduced while tensile strain occurs when the dimension is increased.
Stress and strain are related by a material property called the modulus of elasticity, typically represented by the capital letter “E.” The modulus of elasticity is the stress per unit elastic strain, expressed as a ratio between the stress placed on a material and the resulting strain. The most commonly encountered modulus of elasticity is referred to as Young's modulus; however, “bulk modulus” is also used. Another material property is the Poisson ratio, typically represented by the Greek letter nu, or “ν.” The Poisson ratio compares the transverse strain to the axial strain of a long specimen under an axial tensile or compressive stress at its ends. Stress and strain also combine to produce a “strain energy” equal to the work done during deformation in a manner analogous to the way energy is stored in a spring.
A change in the temperature of a material may result in a deformation of the dimensions of the material. This deformation can be regarded as a thermal strain. The ratio of the change of length per unit length (linear), or change of volume per unit volume (volumetric), for a change in temperature is called the “coefficient of thermal expansion” or CTE. Equivalently, the “thermal coefficient of expansion” or TCE, is typically represented by the Greek letter psi, or “γ.” Thermal strains by themselves generally do not create stress. However, when a material is mechanically constrained from expanding or contracting as a result of the temperature change, it may undergo “thermal stress.”
The general analysis of stress and strain within a multi-layer structure is generally so complicated as to be analytically intractable. See, for example, “An Analysis of an Engineering Model for the Thermal Mismatch Stresses at the Interface of a Uniformly Heated Two Layer Structure,” by L. Matthys and G. De Mey, The International Journal of Microcircuits and Electronic Packaging, Volume 19, Number 3, third Quarter 1996 (ISSN 1063-1674), pp. 323–329, which hereby is incorporated by reference in its entirety into this document. However, where the structure is flat, and where the calculations are conducted far from edge-effect regions in accordance with Saint-Venant's principle, then stress calculations are more tractable and analytic solutions can sometimes be derived. See, for example, Elasticity by J. R. Barber, Kluwer Academic Publishers, 1999, ISBN 0-7923-1610-X (Pb), pp. 34–37, which is also incorporated by reference here.
Thermal stresses and/or strains may be particularly problematic for the operation and/or fabrication of planar optical waveguides. From an optical perspective, stress may degrade performance through a phenomenon called photoelasticity, which results in a problem called birefringence. Briefly, when an isotropic planar waveguide material, such as amorphous silica glass, is subjected to a stress in the plane of the waveguide, the index of refraction in the plane can become different from the index of refraction perpendicular to the plane. This difference produces an effect called “birefringence,” in which an initially linearly polarized optical signal propagating in the plane splits into two polarized rays moving at different velocities, thereby resulting in degradation of the optical signal. Minimizing stress in the optical waveguide minimizes birefringence.
From a mechanical perspective, fracture and warping of the substrate may be problematic. Fracture may be problematic because the materials used in a planar lightwave circuit are often hard and rigid. Thus, rather than deforming plastically, they may fracture suddenly when they reach their ultimate stress limits. Also, they are typically stronger in compression than in tension, and thus tend to fracture easily when subjected to tensile stress.
Warping of the waveguide substrate makes it difficult to use photomasking and etching techniques to define waveguide features on the substrate because warped substrates cannot easily be contacted by flat photomask plates. Warping can include bowing of the substrate in a concave upward direction (dishing) or bowing in a convex upward direction (doming) if the stresses are uniform across the surfaces of the substrate, or a saddle shape (potato-chipping) if the stresses are nonuniform across the surfaces of the substrate. Furthermore, in certain situations (i.e., in high-performance systems in which the waveguide substrate is packaged after fabrication against a second flat substrate), any bumpiness (topography) on the surface of the substrate or any warping of the substrate makes such packaging difficult or impossible.
Thus, to minimize mechanical problems it is desirable to provide a structure, and a fabrication process, in which the substrate is flat (i.e., having low surface topography) and unwarped, the surface materials (especially the thick waveguide cladding materials) are in some degree of compressive stress, and the waveguide core materials experience a stress magnitude too small to produce significant birefringence.
U.S. Pat. No. 4,904,037 to Imoto et al. addresses these problems by providing a waveguide with thermal compensation layers. The device includes a silicon substrate, about 0.4 mm thick, with a thermally grown silicon dioxide film on each side, about 10 μm thick. The silicon dioxide layer on the top side of the substrate is used as a buffer layer and is overlaid with lithographically-defined and etched rectangular optical waveguide cores, about 8 μm thick by 10 μm wide. A silicon dioxide cladding layer (15 μm) is then formed on the front (top) side and a compensation layer (10 μm) having the same composition as the cladding layer may be formed on the rear (bottom) side, so that the total thickness of the set of layers on the front (top) side is 33 μm or less, while the total thickness of the set of layers on the backside is 10 μm or 20 μm. The front side (top side) set is therefore 1.65 to 3.3 times as thick as the back side (bottom side) set. This approach may be of limited usefulness because the topside and bottom side layers have equal coefficients of thermal expansion, and it requires the use of thick layers on the wafer bottom side. In particular, the thermal growth of a silicon dioxide layer 10 μm thick is an unduly lengthy and expensive process, and the growth and deposition of thick layers on the bottom side of the wafer are unduly expensive processes. Furthermore, this approach fails to address problems of warping during the fabrication process, but instead only addresses problems of warping at the end of the fabrication process.
U.S. Pat. No. 5,930,439 to Ohja, et al. describes a planar optical waveguide having at least two silicon dioxide cladding layers on a silicon substrate with at least one silicon dioxide core layer disposed between the cladding layers. The cladding layers have the same refractive index, while the core layer has a higher refractive index than the cladding layers. Ohja, et al. teach that, without doping the overcladding layer to match the CTE of the substrate, it is not possible to achieve polarization sensitivities below 0.1 nm. Ohja, et al. also teach that it is advisable to keep the CTE of the overcladding layer less than that of the substrate so that the overcladding layer stays in a state of compressive stress, a goal which conflicts with the goal of matching the CTE of the substrate. This approach implies an unstated process of engineering optimization in which slight compressive stress is built in to the overcladding layer. This approach fails to address problems of warping during the fabrication process, but instead addresses warping in the finished structure.
Thus, there is a need in the industry for optical waveguide structures that are at the same time economical to fabricate, low in birefringence, and that have substantial compressive stress in cladding layers to reduce fracture tendencies, have low substrate warping at critical points during fabrication, have low surface topography in the final structure, and have low substrate warping in the final structure to allow ease in packaging after fabrication.